Environ. Sci. Technol. 2006, 40, 1154-1162
Long-Term Natural Attenuation of Carbon and Nitrogen within a Groundwater Plume after Removal of the Treated Wastewater Source DEBORAH A. REPERT,† L A R R Y B . B A R B E R , † K A T H R Y N M . H E S S , ‡,| STEFFANIE H. KEEFE,† DOUGLAS B. KENT,§ DENIS R. LEBLANC,‡ AND R I C H A R D L . S M I T H * ,† U.S. Geological Survey, 3215 Marine Street, Suite E127, Boulder, Colorado 80303, U.S. Geological Survey, 10 Bearfoot Road, Northborough, Massachusetts 01532, and U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025
Disposal of treated wastewater for more than 60 years onto infiltration beds on Cape Cod, Massachusetts produced a groundwater contaminant plume greater than 6 km long in a surficial sand and gravel aquifer. In December 1995 the wastewater disposal ceased. A long-term, continuous study was conducted to characterize the post-cessation attenuation of the plume from the source to 0.6 km downgradient. Concentrations and total pools of mobile constituents, such as boron and nitrate, steadily decreased within 1-4 years along the transect. Dissolved organic carbon loads also decreased, but to a lesser extent, particularly downgradient of the infiltration beds. After 4 years, concentrations and pools of carbon and nitrogen in groundwater were relatively constant with time and distance, but substantially elevated above background. The contaminant plume core remained anoxic for the entire 10-year study period; temporal patterns of integrated oxygen deficit decreased slowly at all sites. In 2004, substantial amounts of total dissolved carbon (7 mol C m-2) and fixed (dissolved plus sorbed) inorganic nitrogen (0.5 mol N m-2) were still present in a 28-m vertical interval at the disposal site. Sorbed constituents have contributed substantially to the dissolved carbon and nitrogen pools and are responsible for the long-term persistence of the contaminant plume. Natural aquifer restoration at the discharge location will take at least several decades, even though groundwater flow rates and the potential for contaminant flushing are relatively high.
Introduction Natural attenuation is an approach for groundwater remediation that has been a focus of interest and controversy (1). It has been increasingly used over the last 10-15 years as a * Corresponding author phone: 303-541-3032; e-mail:
[email protected]. † Boulder, Colorado. ‡ Northborough, Massachusetts. § Menlo Park, California. | Current address: U.S. Environmental Protection Agency, Boston, MA. 1154 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 4, 2006
cost-effective means to deal with many types of subsurface contaminants, including hydrocarbons from oil and fuel spills, chlorinated solvents and metals from industrial practices, leachates from wastewater disposal facilities and landfills, radionuclides from weapons production, and explosives from military operations (1-5). The ability of an aquifer to self-remediate depends on many factors, including the type and amount of contamination and the biological, chemical, and physical processes that affect the speciation, transport, and fate of the contaminants. The importance of each process at a specific site depends on the chemical composition of the groundwater, redox conditions within the aquifer, and hydrogeological characteristics of the aquifer (1, 6). Contaminant monitoring is vital for characterizing the effectiveness of natural attenuation. In practice, regulatory requirements usually limit the focus of natural attenuation studies to a rather specific set of toxic contaminants, to those situations in which contaminant levels are exceedingly high, or to a demonstration that contaminant migration has not extended beyond some delineated location downgradient. As a result, there is little information regarding the longterm bulk recovery of an entire groundwater contaminant plume following the removal of the contamination source. The sequence and timing of such a recovery are largely subjects for speculation. Wastewater disposal practices have resulted in large numbers of groundwater contaminant plumes throughout the United States and are an issue of global concern. These include large-volume surface disposals from wastewater treatment plants and small-volume septic systems from individual households and businesses. These plumes typically contain relatively high levels of carbon, nitrogen, and phosphorus, as well as other soluble and sorbed constituents (7-10). Conventional secondary treatment reduces the total amount of organic carbon entering an aquifer relative to septic treatment. However, elevated levels of dissolved organic carbon (DOC) are still seen at many disposal sites and the presence of volatile organic compounds often poses a serious contamination problem (11-12). Behavior of organic contaminants in groundwater varies greatly with respect to degradation and transport, depending on the nature of the compound. Conventional secondary treatment has a minimal effect on total inorganic nitrogen loads entering groundwater, with nitrate more likely to predominate due to nitrification during the treatment process (13). Processes that can affect nitrogen speciation during transport in an aquifer include ammonium sorption/desorption, assimilation, mineralization of organic nitrogen, nitrification, and denitrification. Many of these processes are coupled with or interact with carbon-cycling processes. The unconfined sand and gravel aquifer that underlies most of Cape Cod, Massachusetts is used as a sole-source water supply (14). In many Cape Cod locations, wastewater disposal practices have resulted in groundwater contaminant plumes that affect water quality and the suitability for human consumption (8, 10). In 1995, the source of a large plume of treated wastewater from the Massachusetts Military Reservation (MMR), was removed after more than 60 years of disposal onto rapid infiltration beds. Processes that control the fate and transport of carbon and nitrogen within the MMR wastewater plume have been well-documented (8, 11, 15-18). The purpose of this study was to determine the longterm evolution and dissipation of carbon, nitrogen, and general aquifer geochemistry after the contaminant source had been removed. A 10-year record of the integrated effects 10.1021/es051442j Not subject to U.S. copyright. Publ. 2006 Am. Chem.Soc. Published on Web 01/19/2006
of key processes (hydrological, biological, and chemical) within the contaminated aquifer system provides a basis for defining the general time frame for natural attenuation of wastewater-contaminated aquifers, the sequence of events involved during recovery, and the overall perspective for complete aquifer recovery.
Experimental Section Study Site. The study site is an unconfined, noncalcareous, sand and gravel aquifer located on Cape Cod, Massachusetts. Pristine groundwater at this site is oxic (>300 µM O2), contains low concentrations of total dissolved solids, total nitrogen, nitrate (
